1. Field of the Invention
The present invention relates generally to optical scanners and image forming apparatuses, and more particularly to an optical scanner and an image forming apparatus for scanning a scan object with a light beam from a light source.
2. Description of the Related Art
In image forming apparatuses such as laser printers and digital copiers, a light beam emitted from a light source is modulated according to image information. The light beam passes through a polygon mirror and scan lenses, etc., and focuses on a photoconductor. The light beam scans the surface of the photoconductor in a predetermined direction (main scanning direction) to form a latent image (electrostatic latent image) on the photoconductor. Toner adheres to the latent image to visualize the image information.
In recent years, requirements for faster printing speed have been addressed by enhancing the rotational speed of the polygon mirror and frequencies of clock signals used for modulating light from the light source. However, the printing speed cannot be enhanced sufficiently by these methods. In an attempt to achieve even faster printing speed, a multi-beam method has been devised, which employs plural light sources (for example, see Patent Documents 1 through 3).
The light source employed in the multi-beam method is realized by, for example, a combination of laser chips that each emit a single beam, or an LD array or a vertical cavity surface emitting laser made by incorporating plural light emitting elements into a single laser chip.
These semiconductor lasers such as LD arrays employed in the multi-beam method are widely used as light sources in laser printers, etc., as they are extremely small, and can directly perform modulation at high speed by using a driving current. However, the relationship between the driving current and light output of semiconductor lasers changes with temperature, making it difficult to fix the light intensity of semiconductor lasers at a desired level. Particularly, in vertical cavity surface emitting lasers, light emitting elements are spaced apart by short intervals on a single chip; therefore, the temperature rapidly changes due to light emission and light quenching, and temperature crosstalk occurs significantly, causing variations in light quantity.
Moreover, in a multi-beam optical system including light sources with different oscillation wavelengths, exposure positions of each of the light sources are shifted from each other when color aberrations of scanning lenses are not corrected. As a result, light spots from different light sources are made to scan a medium by different scanning widths, resulting in deterioration in image quality. Accordingly, it is necessary to correct the scanning widths.
For example, in an optical scanner disclosed in Patent Document 1, plural light sources are disposed two-dimensionally, and plural light flux segments from the light sources are deflected by a deflecting unit to scan a medium. Intervals between light emitting points are minimized without causing heat crosstalk between the light emitting points.
Patent Document 2 discloses a method of controlling pixels of electrostatic latent images in an image forming apparatus that employs a vertical cavity surface emitting laser, by changing light emission intensity of each chip by pixel units, and controlling the light emitting time.
Patent Document 3 discloses a method of avoiding heat crosstalk and increasing the density of recorded images in an image forming apparatus employing a vertical cavity surface emitting laser, by prescribing the arrangement of light sources.
In scanning optical systems of image forming apparatuses, polygon mirrors may be tilted, or deflecting reflection surfaces may be disposed at uneven distances from a rotational axis. As a result, light spots (scanning beams) are caused to scan a photoconductor at inconsistent positions and speeds. This leads to fluctuations in the images, which deteriorates image quality.
Over the years, even higher levels of image quality are being demanded. The technologies disclosed in Patent Documents 1 through 3 incur high costs, and are insufficient for achieving the high image quality being demanded.
Patent Document 1: Japanese Laid-Open Patent Application No. 2001-272615
Patent Document 2: Japanese Laid-Open Patent Application No. 2003-72135
Patent Document 3: Japanese Laid-Open Patent Application No. 2001-350111